Not a joke -- I had a similar problem with GDM when migrating to systemd, solved by enabling Avahi. For some freaky reason it is needed.
If it doesn't help you, you can always disable it later, right? Worth a try and few minutes of spare time?

what does "emerge -pv polkit" says?
On my system I had to unmask hard masked (-systemd) flag recently. And yes, GDM has to be emerged with systemd USE flag enabled (then you'll have service file available).

Oddly, I've created both files named in the last two posts, to no avail...
Ideas? Waiting sitting until polkit launches a new version sound temptative at this point, meanwhile leave baselayout, openrc, and sysvinit there just because otherwise I get a broken experience...
I'm using sys-apps/portage-2.1.11.31